1. Field of the Invention
The present invention relates in general to a hydraulically operated brake system of a motor vehicle, and more particularly to an improvement in accuracy of control of the brake pressures for the vehicle wheels to maintain the slip ratios of the wheels within an optimum range.
2. Discussion of the Related Art
An anti- Lock brake control device and a wheel traction control device are known as an apparatus for controlling the brake pressure for a motor vehicle.
The anti-lock brake control device is designed to regulate the brake pressure so as to prevent an excessive slip ratio of the vehicle wheels upon braking of the vehicle. The wheel traction on control device is designed to achieve the same purpose upon acceleration of the vehicle.
An example of such brake pressure control apparatus is disclosed in JP-A-3-128754, which includes wheel speed obtaining means for obtaining the rotating speed of a wheel of the vehicle, and pressure regulating means for regulating the brake pressure to brake the wheel, on the basis of the wheel speed obtained by the wheel speed obtaining means, the running speed cf the vehicle and a predetermined optimum slip amount of the wheel.
The brake pressure is increased or decreased by the pressure regulating means indicated above, which is operated on the basis of the wheel speed obtained by the wheel speed obtaining means. A change in the actual wheel speed actually occurs some time after an increase or decrease in the brake pressure is commenced.
Thus, the known brake pressure control apparatus tends to suffer from a control delay. In view of this drawback, the pressure regulating means is commonly adapted to determine the points of time of increase and decrease of the brake pressure, on the basis of not only the wheel speed but also the wheel acceleration value.
However, the wheel acceleration value is likely to be influenced by external disturbances such as the waviness or bumpiness of the road surface and the vibration of the wheel. The use of a filter for effectively smoothing the wheel acceleration value so as to remove the external disturbances causes a delay in obtaining the wheel acceleration value, and offsets the advantage of introducing the wheel acceleration value for the determination of the points of time at which the brake pressure is increased and decreased.
Thus, it has been difficult to suitably determine the points of time of the brake pressure increase and decrease, and this has been an obstacle to the improvement in the accuracy of control of the brake pressure.